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    <title>Document</title>
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<body>
    <p>不能重复声明</p>
    <script>
        var a = 10;
        var a = 20;
        console.log(a);
        // let b = 10;
        // let b = 20;
        // console.log(b);

        function fn(arg) {
            {
                // 铁男又放大了。。。
                let arg = 999;
                console.log(arg);
            }
            console.log(arg);
        }

        fn(666)
    </script>
</body>
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